How can I have one single Postfix instance routing messages differently,
depends on which IP address they are arriving at SMTP.

If an outside connection arrives, Postfix will deliver message to my
customized transport, which calls a script and passes the message.
If an internal connection arrives, Postfix will deliver message to a
local user's maildir directory.

I know we can configure Postfix to use different content filters based
on different IP, but this is a little different in that I wish to route
message differently.

I noticed the following two "smtp" entry in master.cf. What do they
mean?

smtp inet n - n - - smtpd
smtp unix - - n - - smtp

If I want to accomplish what I wanted, would my master.cf look like the
following, i.e., I have to create my own smtpd deamon process?

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